The broad, long-term goal of this work is to measure kidney nephron and glomerular volume endowment in humans, in vivo. Each nephron contains a glomerulus, which functions as a high-pressure filter of blood macromolecules. Deficits in nephrons and glomeruli have been correlated with renal diseases such as diabetes, obesity, and hypertension. There is currently no noninvasive technique to count the total number of functioning renal glomeruli and nephrons in vivo. Such a technique would enable studies of kidney and systemic disease in humans, and would open a new area of animal studies of the susceptibility to renal disease. We propose that contrast-enhanced MRI, using MRI-detectable nanoparticles targeted to the glomerular basement membrane, can be used to accurately count the total number of glomeruli and measure glomerular volume in the whole, intact kidney. This proposal is focused on establishing this technique in mice. We further propose to assess the accuracy of the technique in the presence of systemic deficits in nephron endowment in mice. Once completed, the proposed work will open the possibility of real-time, in vivo glomerular counts and volume in studies of renal and systemic diseases in animals and humans.